The present invention relates to wafer steppers, and to high-intensity ultraviolet light sources for use in them.
FIG. 1 shows an example of a "stepper" photolithography system. Such systems are commonly used in the manufacture of advanced integrated circuits. In such a system, an illuminator subsystem 100 illuminates a reticle (held in the reticle subsystem 200). The reticle contains a layout pattern for exposure of a single integrated circuit (or sometimes only a portion of an integrated circuit). A lens 300 images the illuminated reticle pattern onto a wafer, coated with photoresist, which is held on the wafer stage subsystem.
At the wafer surface, the dimensions which must be imaged may be well under half a micron. (As of 1993, 0.35 .mu.m is being used in very advanced production processes.) This requires use of short wavelengths. (Typically the 365nm wavelength of a mercury arc, referred to the "I-line," is used for illumination in half-micron systems.) Moreover, to achieve correct photoresist exposure, the image brightness must be extremely uniform over the desired width of the image field. Moreover, to achieve correct photoresist exposure, the image brightness must be extremely uniform over the desired width of the image field. Moreover, to maximize throughput, it is desirable to have a reasonably high image brightness. These desiderata impose very severe constraints on the optical system, and lenses with very aggressive numerical aperture ("NA") values are typically used (often greater than 0.5, and even approaching 0.6 in the newest designs). Additional background on the optics of imaging systems may be found in A. Conrady, APPLIED OPTICS AND OPTICAL DESIGN (2 vols. 1960); R. Kingslake, LENS DESIGN FUNDAMENTALS (1978); R. Kingslake, OPTICAL SYSTEM DESIGN (1983); MICROCIRCUIT ENGINEERING 91 (1992); K. Valiev, PHYSICS OF SUBMICRON LITHOGRAPHY (1992); VLSI TECHNOLOGY (ed. Y. Tarui 1986); the series APPLIED OPTICS AND OPTICAL ENGINEERING; Franon, OPTICAL IMAGE FORMATION AND PROCESSING (1979); and the Optical Society of America's HANDBOOK OF OPTICS (1978); all of which are hereby incorporated by reference.
Lens design to satisfy these constraints is simplified if the light source is quasi-monochromatic (i.e. covers only a narrow range of wavelengths). This is normally done by filtering the light from a mercury arc lamp operated at relatively high power. Thus, a stepper illuminator consists of several optical elements which narrow the bandwidth of the mercury arc lamp illumination prior to reflecting the light down the optical column. Each of these elements reduces the intensity of the illumination, so that most of the optical power output of the arc lamp is necessarily wasted. In order to get more illumination intensity the size of the lamp has been increased from 350 W to up to 1500 W. However, the intensity at the I-line wavelength (365 nm) is already very small when compared to G-line (438 nm), so increasing the size of the lamp does not linearly increase the amount of I-line illumination. The size of the lamp is also limited due to the amount of heat generated, which will degrade the optical component coating. Thus, brightness is very much at a premium in the stepper illumination stage.